This invention relates to an aqueous (meth)acrylate copolymer composition and process for preparation of such composition and the use of such composition in water-borne coatings with improved properties. In particular, this invention is directed to a (meth)acrylate copolymer composition with a hydroxy value above 200, acid value below 50 and weight average molecular weight less than 20,000 comprising a block or graft copolymer prepared directly in water, solvent or water/solvent blend in the presence of a water-soluble macromonomer substantially free of any acid groups so that the final emulsion has a high solids content at low viscosity. This invention also involves coating compositions based on such aqueous copolymer compositions.
Automobiles and trucks receive exterior finishes for several reasons. First, such finishes provide barrier protection against corrosion. Second, consumers prefer an exterior having an attractive aesthetic finish, including high gloss and excellent DOI (distinctness of image). A typical automobile steel panel or substrate has several layers of finishes or coatings. The substrate is typically first coated with an inorganic rust-proofing zinc or iron phosphate layer over which is provided a primer which can be an electrocoated primer or a repair primer. Optionally, a primer surfacer can be applied to provide for better appearance and/or improved adhesion. A pigmented basecoat or colorcoat is next applied over the primer. A typical basecoat or colorcoat comprises a pigment, which may include metallic flakes in the case of a metallic finish. In order to protect and preserve the aesthetic qualities of the finish on the vehicle, it is well known to provide a clear (unpigmented) topcoat over the colored (pigmented) basecoat, so that the basecoat remains unaffected even on prolonged exposure to the environment or weathering. Coating compositions comprise one or more film-forming copolymers and for topcoats acrylic copolymer are preferred. Most commonly, acrylic polymers are linear in structure and cure upon application by reacting with crosslinking agents. The use of non-linear copolymers for coating compositions has also been disclosed, but the use of such polymers, however, have so far found only limited use in the automotive finishes area. See U.S. Pat. No. 5,010,140.
The evolution of environmental regulations has led to the need for products with lower volatile organic content (VOC). However, it is far from trivial to develop aqueous products with desirable properties for automotive finishes. As mentioned above, such finishes must be high performance in terms of aesthetic qualities and durability. Water dispersible polymers are well known in the art and have been used to form waterbased coating compositions, pigment dispersions, adhesives and the like.
The use of cobalt chelates in the preparation of macromonomers for aqueous copolymer dispersions is limited so far to graft copolymers in which either graft or backbone contain ionizable groups in the form of acid or amine. Most of the applications also teach the preparation of such graft copolymers first in a solvent before inverting into a water dispersion.